1. Technical Field
The present invention relates to a continuous casting process and, more particularly, to an improvement in straightening a steel strand in a bow type continuous casting process. The present invention is also related to a bow type continuous casting machine for carrying out this process.
2. Description of Prior Art
Recently, the continuous casting technique, in which molten metal is continuously cast to obtain a strand, has been developed and has replaced the ingot making process followed by rough rolling in the metal industry, including the steel industry. The proportion of steel sections produced by the continuous casting process, in which the steel sections are directly obtained from molten steel by continuously casting such steel, is increasing considerably. The continuous casting process is superior to the conventional ingot making-rough rolling process in that it makes possible high production yield of slabs, blooms and the like and low energy consumption for producing the slabs and the like. This is the reason why the proportion of continuously cast steel sections produced as compared with the ingots is increasing Grades of steels to which the continuous casting process can be applied, have become remarkably varied in recent years.
In the continuous casting process, a hot strand with a liquid core is bent from a vertical direction to a curved shape, and then straightened horizontally. Alternatively, a hot strand with a liquid core is straightened from a bow shape to a horizontal line. After the straightening, the strand is cut to a desired length. However, strain tends to occur in the strand during the bending or the straightening and this results in the formation of defects. The horizontal section of the strand after straightening is not completely solidified and thus includes the liquid core in the modern high speed casting, and, therefore, the generation of (a) bulging strain of the strand caused by the ferrostatic pressure of molten metal (hereinafter referred to as the bulging strain) and (b) strain of the strand caused by straightening at the straightening stage (hereinafter referred to as the straightening strain) results in a very complicated problem which is explained in detail hereinbelow.
Advantageously, the continuously cast and cut strand sections having a great sensible heat are supplied to the rolling stage while the strand sections still retain their great sensible heat, with the result that heat energy and the cost of obtaining rollings can be lessened, as compared with a process in which the strand sections are first heated and then rolled. However, strain is generated in the continuously cast strand due to complicated reasons, and this, in turn causes the generation of cracks on the outer surface and in the interior of the strand in the continuous casting process. Therefore, according to the conventional industrial method, the hot steel sections must be cooled down to room temperature and subjected to the removal of defects prior to being supplied to the rolling stage. In order to make it possible to directly supply the hot steel sections obtained by continuous casting to the rolling stage, the steel sections must obviously be free from internal cracks and must be free from the surface defects, i.e. must not need to be subjected to the removal of surface defects and the like.
the kinds of internal and external defects and the reasons why these defects occur are explained in detail hereinafter. In a widely used continuous casting process, a curved mold for casting the longitudinally curved strand is used so as to keep the height of the continuous casting machine low and thus keep the installation cost low. The height of a continuous casting machine is the vertical distance from the top surface of the mold to a horizontal guiding region for the strand. During the straightening of the longitudinally curved strand, i.e. applying to such strand a bending opposite to the curve of the strand, internal cracks, traversal surface cracks, edge cracks and the like may be generated due to the bulging strain and/or the straightening strain.
One of the conventional technical means to prevent the internal cracks, traversal surface cracks, edge cracks and the like is to arrange the supporting and guiding rollers of the strand, which has left the mold, so that the distances between these rollers is small, thereby causing the bulging amount and strain to be lower. Another technical means is intensive cooling at a secondary cooling zone after the molding takes place which seeks to enhance the hot strength of the solidified shell by, for example, spraying water on the steel at a rate of 1.0 l/kg. Another technical means seeks to keep the straightening strain of a curved strand to a low level and is the straightening method for an unsolidified strand having a liquid core, in which method the straightening strain ranging from 0.1 to 0.25% is distributed over a long straightening region of the strand which is thus made horizontal after undergoing straightening at a plurality of straightening points. This method is hereinafter referred to as the multi-point straightening method. Incidentally, most of the modern continuous casting machines for producing a 200-300 mm thick slab are operated under the following parameters.
Radius of curvature of the basic arc: 10-13 m (a large radius of curvature). PA1 Casting speed: 0.7-2.0 m/minute PA1 Supporting and guiding rollers: the distance between these rollers is small. PA1 Secondary cooling: intensive spray cooling by water. PA1 a curved mold; PA1 a means for supporting and guiding a curved strand withdrawn from the curved mold; PA1 a means for straightening the curved strand at at least two points of the strand; PA1 a secondary cooling means for spraying a mixed medium of gas and liquid on the curved strand within the region of the supporting and guiding means, and the machine having a height of not more than 4.9 m, particularly not more than 3.5 m. The straightening means may be pinch rollers which are arranged in the straightening zones and which define a curve having a plurality of centers of curvatures. The bow type continuous casting machine may further comprise rollers in a horizontal roller zone where small diameter rollers are provided with a small distance therebetween. The strand straightened in the straightening zone is then guided into the horizontal roller zone and conveyed in this zone over an optional length. In the bow type continuous casting machine of the present invention, the solidified shell of a region of the strand within the straightening roll zone, can be made thin due to (a) the low machine height and (b) slow cooling and/or high speed casting.
When the above multi-point straightening method is carried out in these continuous casting machines under the limitation that the machine height (10-13 m) is not increased, the starting point of the multi-point straightening is positioned at a distance from the meniscus in the mold of 15.7-20.4 m as measured along the strand. This distance is determined due to the fact that the machine height of from 10 to 13 m is large. The strand surface temperature and the thickness of the solidified shell at this starting point are from 700 to 900.degree. C. and about 80 to 120 mm (estimated value), respectively. When the strand has a cross section 250 mm thick and 1800 mm wide, the total thickness of the solidified parts of the shell in the thickness direction of the strand amounts to from 70 to 90% of the strand thickness. The strand that is straightened, while the solidified shell has such a thickness, exhibits edge crack defects (in percentage) ranging from 10 to 30% and an evaluation point of the internal cracks (the rate of generating C:1.5) ranging from 4 to 5%, even if the strand is straightened by a straightening means utilizing devices for controlling the straightening force and other devices designed at a modern highly technical level. If a strand having the defects as described above is rolled at the temperature required for rolling, a satisfactorily high yield cannot be obtained.
STAHL UND EISEN VOL 95 (1975), No. 16, p 733-741 describes a process of casting, by means of a curved mold, a strand (average thickness 150 mm) having a radius of curvature of 3.9 m at casting speeds of 0.9 m/min and 0.4 m/min, secondarily cooling the strand by spraying water on it and straightening points. The height of the continuous casting machine is 4.0-4.2 m. The process of STAHL UND EISEN does not seek to provide a strand appropriate for direct rolling; however, the present inventors considered this process in the course of attempting to devise a continuous casting process capable of satisfying the requirements for subsequent use of direct rolling. As a result, the inventors consider that it is difficult in the Stahl und Eisen process to reduce the surface defects to a point so low that the strand can be directly supplied to the rolling stage. This is because the solidified shell at the starting point of the straightening is very thick and, because of this thickness, the allowable limit of straightening strain is low according to the analysis, by the present inventors, of the casing parameters.